From the wording of the verse which requires a Jew to reprove his fellow when he sins (Vayikra 19:17), the Sages derive that one must do so without embarrassing the sinner. Thus, when a person is relating gossip in the company of others, it is preferable that a listener change the subject and later rebuke the speaker privately. One may openly protest gossipmongering in a way that causes the speaker discomfort only if there is no other way to bring his monologue to an end.